A large number of classes of compounds of polyether-polysiloxane copolymers are known. The suitability of each individual type of copolymer for a specific application depends considerably on the structure of the polysiloxane and polyether units, their ratio, and their arrangement relative to one another, and the type of linkage between these units. In this connection, the linking of the units can be distinguished, inter alia, by Si--O--C bonding or by Si--C bonding.
As is known, polysiloxane-polyether copolymers linked by Si--O--C bonds are prepared by reaction of OH--terminated polyethers with siloxanes containing halogen, hydroxyl, alkoxy or acetoxy groups, for example (cf. W. Noll, "Chemie und Technologie der Silicone", Verlag Chemie, 1968, p. 321). These polyether-polysiloxane copolymers are known and available commercially.
Si--C-linked polysiloxane-polyether copolymers can, for example, be obtained by reaction of alkenyl-containing polyethers with polysiloxanes which contain Si--H groups with the help of a substance which accelerates the reaction of alkenyl compounds with Si--H bonds, e.g. Pt catalysts. For example, U.S. Pat. No. 5,625,023 describes compounds which are intended to prevent the formation of aerosols in silicone-based coatings which are to be applied quickly. These compounds are obtained by reacting an organohydrogensilicon compound with an oxyalkylene compound, a catalyst and a monohydric olefinic alcohol. The reaction can in this connection be carried out, inter alia, by hydrosilylation or by the addition reaction of the side and/or terminal SiH groups with the polyethers terminated at both ends with alkenyl groups, forming Si--C bonds between polyether and siloxane. The use of silicon compounds having Si--H side groups leads to branched copolymers. Using the process described it is not possible to prepare exclusively linear polyether-polysiloxane copolymers of defined molecular size.
EP 643 329 claims a large number of polyoxyalkylene-polysiloxane block copolymers for liquid radiation-curable compositions. In the general formula (II), Si--C bonds are present between polyether and polysiloxane units, one polysiloxane unit being a block in the middle of the molecule flanked by polyether groups, rather than two or more blocks or units being randomly distributed throughout the molecule.
DE 30 11 304 and DE 31 23 103 describe antifoam formulations for use in aqueous dispersions and solutions of synthetic resins which are free from mineral oxides, such as SiO.sub.2 and Al.sub.2 O.sub.3, and contain polysiloxane-polyether block polymers. These polymers, containing alternating polysiloxane and polyether units, are linked both via Si--C bonds and via Si--O--C bonds. In addition, DE 38 07 247 describes the use of polyoxyalkylene-polysiloxane block copolymers for the defoaming of aqueous dispersions, polysiloxane and polyoxyalkylene being linked exclusively via Si--O--C bonds.
Polysiloxane-polyether block copolymers in which polyether and polysiloxane units are linked via Si--O--C bonds have the disadvantage that these linkage sites can be cleaved in the presence of compounds containing free hydrogen atoms, such as, for example, alcohols, polyols or water (cf. W. Noll, "Chemie und Technologie der Silicone" [Chemistry and Technology of Silicones], Verlag Chemie, 1968, p. 323), the cleavage being accelerated considerably in particular by the presence of acidic or basic compounds. For example, in aqueous systems this leads to the hydrolysis, and in alcoholic systems to the alcoholysis of the polysiloxane-polyether block copolymers. This effect occurs in particular during storage of formulations which comprise such polysiloxane-polyether block copolymers and compounds containing free hydrogen atoms, and thus leads to an undesired negative change in the properties.